1. Field of the Invention
The present invention relates to a position measurement device using GPS (global positioning system), a portable terminal provided therewith, and a position measurement method.
2. Description of the Related Art
With GPS, the earth is surrounded by 24 satellites; the distance from those satellites is measured and the location of the measurement position on the earth is calculated. Currently, GPS has a wide variety of uses such as in car navigation systems and surveying.
Meanwhile, among mobile bodies, cellular phones are spreading at an increasing rate and are used for emergency notification. It would be desirable to include a GPS function therein for that purpose, to be able to specify the location at which the emergency is occurring, and for businesses"" service deployment with the knowledge of customer locations.
With car navigation systems, there are limits that the location at which power is cut off is generally same location where power is turned on, and that the vehicles do not travel off the roads. With cellular phones, on the contrar, it may often be impossible to ensure the necessary number of satellites for GPS positioning because of the cellular phones being carried to locations from which is difficult to see the sky, because of their high level of portability, and the inclusion of GPS functions in cellular phones has been obstructed.
A minimum of four satellites must be supplemented acquired in order to position a location. Three are necessary even when the altitude is set at the surface of the earth. In other words, the position cannot be calculated from a location with no direct line of sight to the minimum of three satellites. For this reason, position measurement cannot at present be done from streets with many tall buildings.
Car navigation systems or the like shift to a self-navigation mode when positioning with GPS becomes difficult. This self-navigation mode is a positioning method for calculating the distance and direction of displacement from the nearest position measured by GPS using an inclination sensor for finding the angle of the vehicle, a vehicle speed sensor for detecting the speed from the rotation of the tires, and a uniaxial gyro for detecting displacement to either side.
However, in the case of portable terminals (mobile bodies) including cellular phones, unlike car navigation systems, positioning is not possible with the abovementioned self-navigation mode in car navigation systems since the mode of conveyance of the portable terminal is not fixed as with a vehicle, and can be directed vertically and horizontally. When the number of satellites necessary for measuring the portable terminal position with GPS cannot be ensured, it becomes necessary to include separate positioning means, different from the abovementioned self-navigation mode in car navigation systems, in the portable terminal.
Consequently, it is an object of the present invention to provide a position measurement device, a portable terminal provided therewith, and a position measurement method, so that positioning is possible even when the number of satellites necessary for GPS positioning cannot be ensured, in the case where a position measurement device using GPS is included in a portable terminal.
In order to achieve the abovementioned object, a first position measurement device relating to the present invention measures the position by finding the displacement distance from the nearest position on the basis of the three-dimensional acceleration detected by a three-dimensional acceleration sensor, for example, in the case where the number of satellites necessary for position measurement cannot be acquired.
Furthermore, a second position measurement device relating to the present invention measures the position by finding the displacement distance from the nearest position on the basis of the three-dimensional acceleration detected by a three-dimensional acceleration sensor, for example, and also by finding the displacement direction from the nearest position on the basis of the three-dimensional angular velocity detected by a three-dimensional gyro, for example, in the case where the number of satellites necessary for position measurement cannot be acquired (especially in the case where not even one of the satellites necessary for position measurement cannot be acquired).
Also, the present invention provides a method for determining the presence of an external magnetic field, for determining the presence of an external magnetic field other than geomagnetism on the basis of a comparison of magnetic charges detected by magnetism sensors (azimuth sensors) disposed in mutually different locations.
Furthermore, a method for determining the presence of an external magnetic field is provided, wherein a database of geomagnetic charges (reference geomagnetic charges) of measured positions is established in advance, and which determines the presence of an external magnetic field other than geomagnetism on the basis of a comparison of a magnetic charge detected by a magnetism sensor (azimuth sensor) with a reference geomagnetic charge at that location.